1. Field of the Invention
This invention relates to a vascular prosthesis which is fabricated in a tubular shape of fibers and a method for the production thereof. More particularly, this invention relates to a vascular prosthesis which excels in elasticity and possesses characteristic qualities similar to those of a natural blood vessel capable of allowing ideal attachment thereto of cells and tissues and a method for the production thereof.
2. Description of the Prior Art
As the substitutes for natural blood vessels, the vascular prostheses made of woven and knit fabrics of polyester and expanded polytetrafluoroethylene have already been commercialized to date.
These vascular prostheses are directly implanted in patient's bodies and left standing therein for attaching cells and tissues thereto. Otherwise, at first in vitro vascular cells collected from host's natural blood vessels under an aseptic condition are attached to the vascular prostheses, then the cells attached vascular prostheses are implanted in patient's bodies and left standing therein for causing biological reactions of the attached cells and further attaching cells and tissues thereto.
For such vascular prostheses, what is important is the improvement in their patency. This requirement constitutes itself a particularly important task imposed on the development of a vascular prosthesis of a small diameter usable for transplantation of a peripheral artery. As one measure to improve the patency of a vascular prosthesis, the impartation to the artificial blood vessel of a physical property (elasticity) similar to that of a natural blood vessel subjected to transplantation may be cited (Waden R. et al.: Matched Elastic Properties and Successful Arterial Grafting, "Arch Surg.," Vol. 115, October 1980, pp. 1166 to 1169). In order that the patency of a vascular prosthesis particularly of the type having a small diameter may be enough to preclude thrombus on the inner wall surface of the vascular prosthesis and ensure the uniformity of the blood flow inside the anastomosed vascular prosthesis graft and the natural blood vessel, the vascular prosthesis is desired to possess a similar physical property (elasticity) with that of the corresponding natural blood vessel. Due to mismatch of physical property between the artificial and the natural blood vessel, the blood flow is disturbed possibly to the extent of causing thrombus particularly at the site of anastomotic end and consequently inducing intimal hyperplasia at the site of anastomotic end. It is held that the thrombus and the hyperplasia govern the lasting patency of the vascular prosthesis (Kinley CE et al.: Compliance: A continuing problem with vascular grafts, J. Cardiovas Surg 21: 163-170, 1980 or Okuhn SP et al.: Does compliance mismatch alone cause meointimal hyperplasia? J. Vascular Surg 9: 35-45, 1989).
As a way of imparting to a given vascular prosthesis the similar physical property with that of the natural blood vessel, the approach resorting to use of an elastic material has been in vogue. To be specific, a method which comprises preparing polyurethane as an elastic polymer and forming porous structure (Japanese Unexamined Patent Publication No. 182,958/1985, Japanese Patent Publication No. 11,861/1987, and Japanese Unexamined Patent Publication No. 183,757/1987), a method which comprises forming a multilayer porous structure by electrostatic spinning (Japanese Patent Publication No. 11,861/1987), and a method which comprises superposing porous sheets of nonwoven fabric (Japanese Unexamined Patent Publication No. 183,757/1987) have been proposed.
Further, for such vascular prostheses, the important matter is whether or not cells and tissues naturally existing in a host's body or cells and tissues extracted from a host's body and seeded are successfully attached to the outer and inner wall surfaces of the vascular prosthesis and allowed to gain in growth and eventually form a similar histologic structure with that of the host's own blood vessel.
For the purpose of ensuring fast attachment of the cells and tissues ideally, the practice of enhancing growth of the cells and tissues on the wall surfaces of the vascular prosthesis by forming loops on the wall surfaces of the vascular prosthesis which is made of fibers or by forming a finely porous texture in the wall of the vascular prosthesis which is made of a synthetic resin tube has been known.
For the formation of loops on the wall of a vascular prosthesis which is formed of a tubular fabric, for example, a method which, as disclosed in Japanese Patent Publication No. 14,823/1986, effects this formation of the loops in the production of the tubular fabric with a double raschel knitting machine by causing loop-forming yarns to be fed to the knitting machine at a larger speed than lattice-forming yarns has been known.
Another method which, as disclosed in Japanese Patent Publication No. 52,898/1988 covering an invention aimed at enhancing the proof of a knit fabric against fraying, accomplishes the formation of loops on the walls of a tubular fabric by subjecting this tubular fabric to a treatment with a high-pressure fluid thereby causing at least part of fibers intended for forming fluffs and/or loops to be twisted to fibers destined to form a matrix structure has been known.
When the vascular prosthesis is made of polyurethane, a substance which is capable of imparting to the product a similar physical property (elasticity) with that of a natural blood vessel, the produced vascular prosthesis indeed acquires an outstanding elasticity. However, when the polyurethane product is implanted to the body and left for a long period, the susceptibility of the polyurethane product to deterioration of the physical property has come to be unveiled (Tabata et al.: Clinical and experimental study on the insulation damage of polyurethane pacemaker leades, Jpn J Artif Organs 19(4), 1427-1431 (1990)).
The vascular prosthesis is an artificial internal organ which by nature is required to function permanently in vivo and is known to be inherently prone to the impact of water hammer stress arising from the harsh pulsative flow of blood. It must avoid deterioration in quality because this deterioration could cause aneurysm or dilation.
In accordance with the method disclosed in Japanese Patent Publication No. 14,823/1986 which comprises formation of loops on the wall surfaces of a vascular prosthesis, the produced vascular prosthesis has the disadvantage that the number of loops per unit surface area of the wall is small because one cycle of the needle motion produces only one loop. This vascular prosthesis, therefore, cannot sufficiently allow fast attachment of cells and tissues to the walls thereof. In accordance with the method disclosed in Japanese Patent Publication No. 52,898/1988, no sufficient intertwisting of fibers is obtained if the pressure of injection of the high-pressure flow is unduly small, and unnecessary breakage of surface fibers ensues and overall pressureproof of tube declines if this pressure is unduly large. Moreover, this method has the disadvantage that adjustment of the pressure of injection is not easy, application of uniform pressure on tubular fabric is difficult, and formation of loops is liable to result in disruption of uniformity of production. This method, therefore, encounters difficulty in obtaining a vascular prosthesis possessed of desired characteristic properties.
Improvements have been given to vascular prostheses which are made of a woven or knit fabric of polyester fibers represented by Dacron (Du Pont's proprietary product) and impartation of softness to such vascular prostheses has been accomplished successfully by adoption of extremely fine fibers. In the vascular prosthesis formed of these polyester fibers, impartation of elasticity has not been realized.